Optical telecommunication systems require that a given signal be efficiently transmitted from an origination point to an intended termination point, while maintaining a signal quality that is suitable for use of the signal upon its delivery. The ever present drive for lower costs, higher bandwidth, and faster service creates challenges to the maintenance of acceptable signal quality. Meanwhile, signal quality demands also simultaneously evolve, creating increased system performance requirements that often are in conflict with the drive for lower costs.
One of the many prevalent signal quality issues concerns signal failure protection. A signal transmitted over a particular path is vulnerable to failure of the equipment supporting the path. To address this vulnerability, a copy of the signal can be provided for transmission by a different path. Although failure protection by providing such signal copies is broadly desirable, there are costs associated with implementation of such protection. For example, splitting of the original signal into two separate signals typically requires either the use of additional resources to generate and provide energy to carry the second signal, or acceptance of the attenuation resulting from direct signal splitting.
Another signal quality issue concerns chromatic dispersion. A typical signal is transmitted within a narrow band of wavelengths brackets a desired center wavelength, which wavelengths nevertheless travel at slightly different speeds. On an extended signal transmission path, these different speeds cause a given signal pulse to spread out in time. This pulse spreading, also referred to as chromatic dispersion, can result in partial overlapping of adjacent pulses within the signal. Herein, a bit signified by the presence of pulse energy is said to be “asserted,” whereas a bit signified by the absence of pulse energy is said to be “unasserted.” Although an asserted bit is often interpreted as having the value “one” and an unasserted bit as having the value “zero,” other interpretations are also possible. If constructive interference occurs in the overlap region between adjacent pulses, a portion of the original signal intended to register as a zero or unasserted data bit, can be distorted to incorrectly register as a one or an asserted data bit.
There is thus a need for apparatus capable of generating improved telecommunication signals that are provided with failure protection as well as resistance to chromatic dispersion, and a need for methods capable of generating, transmitting and receiving such signals.
In particular, such apparatus and methods are needed in the context of non return to zero (NRZ) coding, in which mutually adjacent asserted data bits are “conjoined;” that is, the pulse energy does not return to zero between such bits, but instead, those bits are directly joined together into a unitary asserted data bit sequence having a length equivalent to the cumulative data bit lengths that are so joined together.